Current communication systems commonly involve high data transfer rates of more than 5 Gbps. In such systems, cross-talk between signal lines is a major concern.
In an ideal electrical interconnection system, the system will carry signals without suffering from signal distortions such as cross-talk. Cross-talk occurs when one signal creates an unwanted signal on another signal line. Generally, cross-talk is caused by electromagnetic coupling between signal lines. Cross-talk is a particular problem for high-speed, high-density electrical interconnection systems. Electromagnetic coupling increases when adjacent signal lines are in close proximity or when signals being carried are higher in frequency. Both of these conditions are present in high-speed, high-density electrical interconnection systems. Discontinuities in the connectors often contribute to, and exacerbate, the cross-talk problems.
To reduce crosstalk signal distortion, a number of approaches are commonly used. For example, a shielding can be incorporated into the connectors in the connection system, or differential signals can be used in adjacent signal lines for transmitting information. One differential signal is carried on two conductors, with the signal being represented as the difference in electrical levels between the conductors. A differential signal is more resistant to cross-talk than a single-ended signal, because any stray signals impinging on the conductors will generally change the level on both conductors, but do not alter the difference in levels.
Consequently, conventional high-speed I/O connection cable assemblies use circuit boards as a plug body to be plugged into a jack connector and cables having a pair of wires for carrying the differential signal. The printed circuit board has traces and pads on at least one of its surfaces, wherein particular contact pads may be contacted by a mating receptacle. The traces transmit electrical signals across the printed circuit board. For example, they may transmit signals from the contacts of the mating receptacle to the wires of the plug assembly and vice versa.
The plug assembly may include several pairs of wires, such as four input signal cables and four output signal cables. However, the plug assembly may also include up to 32 or more pairs of wire, where the plug assembly includes wire pairs in increments of two. Often the plug assembly further includes drain wires which are connected to the shielding of each cable.
The transmission between the printed circuit board and the cable occurs through a connector, which connects the printed circuit board and the cable together. The connector therefore should also to be capable of handling the high data rates, in addition to preventing cross-talk signal distortion. With modern intercommunication systems becoming smaller in size, the size of connector is also being reduced. As the connector becomes smaller, the space between adjacent cables is also being reduced, which increases the chances of undesired cross-talk between the adjacent cables. Consequently, a major source of cross-talk is at the connector located at the printed circuit board and cable interface.
In order to reduce cross-talk on the connection interface element, it is known to separate the input and output pairs of wires (Rx and Tx) by routing the input and output pairs on the top and bottom of the printed circuit board, respectively. By employing additional ground layers within the printed circuit board, cross-talk between Rx and Tx lines can be eliminated effectively.
However, at the interface whereto the cable is soldered, impedance compensation is used in order to avoid distortions of the signal due to signal reflection. Therefore, conventional cable assemblies dispense with internal ground layers in the particular region where the cables are soldered to the conductive pads of the traces. However, by reducing impedance through this method, cross-talk signal distortion between the signal lines is no longer eliminated effectively. In particular, when complying with 10 Gbps specifications, such as IEEE standard 802.3ap 10GBASE-KR (2008), balancing impedance reduction with cross-talk reduction by these methods can result in unsatisfactory levels of both. Consequently, there is a great need to develop smaller connectors that effectively reduce both impedance and cross-talk at the same time.